A choke valve is a throttling device commonly used as part of an oil or gas field wellhead. It functions to reduce the pressure of the fluid flowing through the valve. Choke valves are placed on the production “tree” or “manifold” of an oil or gas wellhead assembly to control the flow of produced fluid from a reservoir into the production flow line, and is used on wellheads or manifolds located on land, offshore, or beneath the surface sub-sea of the ocean (sub-sea). Examples of choke valves used in oil and gas fields are generally described in U.S. Pat. No. 4,540,022, issued Sep. 10, 1985, to Cove and U.S. Pat. No. 5,431,188, issued Jul. 11, 1995, to Cove. Both patents are commonly owned by the applicant of this application, Master Flo Valve, Inc.
In general, choke valves include:
a valve body having an axial main bore, a body inlet (extending along an inlet bore, typically oriented as a side outlet to the axial main bore) and a body outlet (extending along an outlet bore, usually aligned with the axial main bore);
a “flow trim” mounted in the main bore between inlet and outlet, for throttling the fluid flow moving through the body; and
biassing members such as a stem and bonnet assembly for actuating the flow trim to open and close the choke valve, and for closing the upper end of the axial main bore remote from the outlet.
There are four main types of flow trim commonly used in commercial chokes or control valves, each of which includes a port-defining member forming one or more flow ports, a movable flow control member for throttling the flow ports, and seals to implement a total shut-off. These four types of flow trim can be characterized as follows:
(1) a needle and seat flow trim comprising a tapered annular seat fixed in the valve body and a movable tapered internal plug for throttling and sealing in conjunction with the seat surface;
(2) a multiple-port disc flow trim, having a fixed ported disc mounted in the valve body and a rotatable ported disc, contiguous therewith, that can be turned to cause the two sets of ports to move into or out of register, for throttling and shut-off;
(3) a cage with internal plug flow trim, comprising a tubular, stationary cylindrical cage, fixed in the valve body and having ports in its side wall, and an internal plug movable axially through the bore of the cage to open or close the ports. Shut-off is generally accomplished with a taper on the leading edge of the plug, which seats on a taper carried by the cage or body downstream of the ports; and
(4) a cage with external sleeve flow trim, comprising a tubular, stationary cylindrical cage having ports in its side wall and a hollow cylindrical external sleeve (also termed external flow collar) that slides axially over the cage to open and close the ports. The shut-off is accomplished with the leading edge of the sleeve contacting an annular seat carried by the valve body or cage.
In each of the above, the flow trim is positioned within the choke valve at the intersection of the valve's inlet and outlet. In the latter two types of valves, termed “cage valves”, the flow trim includes the tubular, stationary cylinder referred to as a “cage”, positioned transverse to the inlet and having its bore axially aligned with the outlet. The cage has one or more restrictive flow ports extending through its side wall. For cage valves, flow through the ports of the cage is controlled by a flow control member which is either an internal plug component, or an external sleeve/flow collar component. Fluid enters the cage from the choke valve inlet, passes through the flow ports and changes direction to leave the cage bore through the valve outlet.
The valve body is formed of softer material, typically steel, while the flow trim components are typically manufactured from a hardened, high wear material such as tungsten carbide. The steel body is machined in the course of fabrication and must cope with stresses, and thus is manufactured from a relatively ductile steel. The flow trim however has harder surfaces. Typically the cage of the flow trim is formed of tungsten carbide, the internal plug is formed of tungsten carbide, and a tungsten carbide liner is shrink-fitted as a liner in flow collar. This is important because the flow trim is positioned at the bend of the “L”, where it is exposed to, and temporarily contains, the fluid flow when it is accelerated, is changing direction, and is in a turbulent state. Erosion of the flow trim may be extreme, causing catastrophic failure of the choke valve, which results in over pressurization of the downstream equipment or damage to the well formation due to excessive flow.
Production interruptions occur with surface and sub-sea facilities when there is erosion (i.e., wear) of the valve flow trim to the point that the flow trim needs to be replaced. It is important to replace the flow trim before damage to other valve internals or the valve body is allowed to occur. Depending on the application and choke operation conditions, erosion can occur in different locations of the flow trim, including the cage ports, the outlet end of the cage, the flow collar liner or the upstream end of the plug (i.e., opposite or facing the outlet). Wear detection and signalling to notify of flow trim wear, while needed, is an inexact technology for many reasons. Wear of the flow trim occurs at different locations and to different degrees, depending on the application and choke operating conditions. Most areas of the flow trim such as the ports, the plug or the flow collar are not accessible and/or hospitable locations for sensors or transmitters to be inserted. As well, once erosion starts to occur, the rate of erosion may accelerate to the point that the flow trim, and thus the choke, can fail in a very short period of time.
There is thus still a need for erosion monitoring for choke valves of the cage valve types. Examples of cage valves with external flow collar are shown in, for instance, U.S. Pat. No. 4,540,022, issued Sep. 10, 1985, to Cove et al., U.S. Pat. No. 6,105,614, issued Aug. 22, 2000 to Bohaychuk et al., and U.S. Pat. No. 7,426,938, issued Sep. 23, 2008 to Bohaychuk et al. A choke valve including an external flow collar flow trim in sub-sea applications is shown in U.S. Pat. No. 6,782,949 to Cove et al. These patents describe the beneficial characteristics of the external sleeve/flow collar design in erosion control, valve outlet erosion protection, seating integrity, and fluid energy control features. An exemplary choke valve including an internal plug flow trim component is shown in US Patent Publication No. 2010/0288389 A1 to Hopper et al., and assigned to Cameron International Corporation.
FIG. 1 shows a typical prior art choke valve in which the flow trim includes an external tubular throttling sleeve (flow collar) that slides externally over the side wall of the cage. The sleeve acts to reduce or increase the area of the flow ports. An actuator, such as a threaded stem assembly, is provided to bias the sleeve back and forth along the cage. The rate that fluid passes through the flow trim is dependent on the relative position of the flow collar on the cage and the amount of port area that is revealed by the sleeve.
In sub-sea wellheads, maintenance cannot be performed manually. An unmanned, remotely operated vehicle, referred to as an “ROV”, is used to approach the wellhead and carry out maintenance functions. To aid in servicing sub-sea choke valves, such choke valves have their internal components, including the flow trim, assembled into a modular sub-assembly. The sub-assembly is referred to as an “insert assembly” and is inserted into the choke valve body and clamped into position. FIG. 2 shows a typical prior art sub-sea choke valve with flow trim of the external throttling sleeve (flow collar) type.